1. Field of the Invention
This invention relates to a drill bit comprising a drill and a drill holder for holding the drill, a drill tool provided with the drill bit, and a manufacturing method of the drill bit.
2. Description of the Related Art
A drill bit is known, for instance as disclosed in Japanese Examined Patent Publication No. 8-18213/1996, comprising a drill part formed with a spiral bit on a circumference thereof and a holding part with a larger diameter than the drill part. The drill bit is drivingly rotated in a state that the holding part is held by, for instance, an electric drive source.
There has been a common practice, when manufacturing the drill bit, to shape the drill part and the holding part as one-piece unit by machining one metallic material with a lathe or the like.
However, the above machining process is cumbersome and requires a lot of processing, leading to a remarkable production cost rise.
As a means to solve the above problem, several drill bits have been proposed, as shown in FIGS. 16A to 16C. In any of these prior art drill bits, the drill bit comprises a drill corresponding to the drill part and a holder corresponding to the holding part and is constructed such that a shank formed at a base end of the drill is inserted into an insertion hole formed in the holder to joint the drill together with the holder. The specific construction of these drill bits is described one by one with reference to FIGS. 16A to 16C.
A drill bit 100 shown in FIG. 16A has a drill 200 including a main part 201 formed at a lead end thereof and a cylindrical shank 202 formed at a base end thereof. The drill main part 201 and the shank 202 are integrally formed. A spiral bit 203 is formed on the drill main part 201, and an external thread 204 is formed at an end of the shank 202.
The drill bit 100 has a holder 300 shaped into a cylinder with a larger diameter than the drill 200. A screw hole 301 is formed inside a lead end of the holder 300 concentrically with an axis of rotation of the drill 200. Screwing the external thread 204 of the shank 202 into the screw hole 301 enables mounting the drill 200 onto the holder 300.
The drill bit 101 shown in FIG. 16B is constructed by inserting a shank 202 into an insertion hole 311 formed in a holder 310 and adhering the joint part of the shank 202 and the insertion hole 311 with a brazing filler metal 312 including silver and nickel alloy, in place of providing the external thread 204 and the screw hole 301.
A drill bit 102 shown in FIG. 16C is constructed such that a flat part 204 is formed on an outer circumference of a shank 202, and a press fit hole (screw hole) 322 is formed extending from an outer circumference of a holder 320 toward a shank insertion hole 321 to communicate with the flat part 204. In this arrangement, inserting the shank 202 into the shank insertion hole 321 so as to set the flat part 204 in the press fit hole 322 and press fitting a metal piece 400 into the press fit hole 322 (or screwing an external thread 400 into the screw hole 322) enables preventing relative rotation of a drill 200 to the holder 320.
The drill bit 100 shown in FIG. 16A is constructed by screw joint of the drill 200 with the holder 300. Accordingly, the joint state may be loosened due to vibration during working with a result that the drill 200 unnecessarily vibrates in spite of holding of the holder 300 and in a worst case, the drill 200 may come out from the holder 300.
In the case of the drill bit 101 shown in FIG. 16B, if a large torque acts upon the drill bit 101 during working, adhesion of the joint part may be weakened, leading to breakage of the joint part. In such a case, the drill 200 may idly rotate relative to the holder 310 (namely, the torque is not transmitted to the drill 200), making the drill bit 101 usable. Particularly, filling of the filler 312 between the shank 202 and the insertion hole 311 is insufficient (poor filling state), even application of a small torque may break the joint part. This is not desirable when seeking for reliability of the drill bit.
The construction of the drill bit 102 shown in FIG. 16C also does not provide an effective solution to prevent relative rotation of the drill 200 to the holder 320 because there is a possibility that the metal piece (or external thread) 400 may be loosened and come out from the press fit hole (or the screw hole) 322 due to vibration and centrifugal force generated during working.
Other fixation means such as close fitting the shank 202 in the insertion hole 311 by shrinkage fit or press fit can be applied. However, such fixation by a close fit cannot sufficiently resist a large torque generated when start the drill bit starts rotating, and the problem that the drill rotates relative to the holder cannot be avoided.